1. Field of the Invention
This invention pertains broadly to the field of acoustic wave systems and devices. More particularly, but without limitation thereto, the invention pertains to a device that converts inaudible sounds of particular phase relationships into audible sounds of substantially the same phase relationships.
2. Description of the Prior Art
In the process of ordinary listening, it is possible not only to detect sounds but also t surmise from where sounds emanate. Sound detection requires only a single receptor while determining the direction of a sound's source requires two or more receptors placed at locations distant from each other.
In the familiar case of human hearing our ears permit us to ascertain the direction from which a sound is emitted. The convolutions of the ear in conjunction with the ear drums serve to tell us upon which ear a sound is more intense. A sound emanating closer to one earwwill not only appear louder to that ear but will impinge upon the closer ear first. In this sense both the intensity as well as the time-of-arrival of the sound will cue a listener as to the direction from which the sound emanates.
Sound within the audible hearing range can be heard outright if the listener is close enough to the sound or can be heard from greater distances if the listener utilizes typical amplification techniques. In order for humans to hear sounds above or below the audible range, however, it is necessary to trans)ate these sounds to a band of frequencies perceivable by the ordinary human ear.
Prior art exists for translating inaudible frequencies into frequencies of the audible sound range. In FIG. 1 there is shown a typical frequency translator 10. Incoming baseband signals 12, such as ultrasonic signals, are mixed in a mixer 14 with a local reference frequency 16 generated by a local oscillator 18.
The resulting output of translator 10 is described graphically in FIG. 2. The mixing or heterodyning process creates sum 20 and difference 22 frequency bands of the 24 original baseband 12 (f.sub.1 through f.sub.2).
FIGS. 1 and 2 show how a desired translated frequency band can be preserved by utilizing common filtering techniques. In the case of ultrasonics, the audible frequency-downshifted difference-component 22 can be preserved by means such as low pass filter 24. In this instance, filter 24 has a response curve 26 that prevents frequency components outside of the curve from passing to a listener.
In the prior art technique shown in FIG.-, local oscillator 18 free-runs to continuusly generate reference frequency 16. As a consequence, the phase of any translated signals with respect to the phase of the incoming signals is random. This is apparent by viewing FIG. 3 which shows how the local oscillator reference frequency 16 used in this prior art frequenoy translation process can easily be out-of-phase with received baseband signals 12.
The prior art system described works well where only a single channel of signal reception is being processed and where the phase of the translated output signal is of no consequence. To pinpoint the direction from which a sound emanates, however, at least two channels must be used with the phase of the incoming sounds being preserved.